System and method for tracking medical articles

ABSTRACT

An automatic data collection system tracks medical articles by providing a robust electromagnetic (EM) field within an enclosure in which the articles are stored. Respective data carriers, such as RFID tags, attached to each article respond to the electromagnetic field by transmitting data identified with each article. An RFID scanner receives the transmitted RFID tag identification data and a processor compares the received identification data to a data base. The data base associates the identification data with data concerning the medical article to which the RFID tag is affixed, such as the name of the medicine, the size of the dose, and the expiration date. The processor is also programmed to keep track of the number of articles of a particular type remaining in the enclosure, to note receipt of an article in the enclosure, and to note removal of the article.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/691,518, filed Apr. 20, 2015, now U.S. Pat. No. 9,223,934, which is acontinuation of U.S. application Ser. No. 14/231,513, filed Mar. 31,2014, now U.S. Pat. No. 9,013,309, which is a continuation of U.S.application Ser. No. 13/776,613, filed Feb. 25, 2013, now U.S. Pat. No.8,686,859, which is a continuation of U.S. application Ser. No.12/631,861, filed Dec. 7, 2009, now U.S. Pat. No. 8,384,545, all ofwhich are incorporated by reference in their entireties.

BACKGROUND

The invention relates generally to the field of automatically collectingdata from articles being tracked, and more particularly, to a system andmethod of establishing a robust electromagnetic field in an enclosurefor use in identifying tagged articles.

There are a number of ways of identifying and tracking articlesincluding visually, optically (bar coding, for example), magnetically,RFID, weighing, and others. Where an automatic system for tracking isdesired, RFID is a candidate since identification data may be obtainedwirelessly. RFID tags have decreased in cost, which has made them evenmore attractive for such an application.

Radio-frequency identification (“RFID”) is the use of electromagneticenergy (“EM energy”) to stimulate a responsive device (known as an RFID“tag” or transponder) to identify itself and in some cases, provideadditionally stored data. RFID tags typically include a semiconductordevice having a memory, circuitry, and one or more conductive tracesthat form an antenna. Typically, RFID tags act as transponders,providing information stored in the semiconductor device memory inresponse to an RF interrogation signal received from a reader, alsoreferred to as an interrogator. Some RFID tags include securitymeasures, such as passwords and/or encryption. Many RFID tags alsopermit information to be written or stored in the semiconductor memoryvia an RF signal.

RFID tags may be incorporated into or attached to articles to betracked. In some cases, the tag may be attached to the outside of anarticle with adhesive, tape, or other means and in other cases, the tagmay be inserted within the article, such as being included in thepackaging, located within the container of the article, or sewn into agarment. The RFID tags are manufactured with a unique identificationnumber which is typically a simple serial number of a few bytes with acheck digit attached. This identification number is incorporated intothe tag during manufacture. The user cannot alter thisserial/identification number and manufacturers guarantee that eachserial number is used only once. This configuration represents the lowcost end of the technology in that the RFID tag is read-only and itresponds to an interrogation signal only with its identification number.Typically, the tag continuously responds with its identification number.Data transmission to the tag is not possible. These tags are very lowcost and are produced in enormous quantities.

Such read-only RFID tags typically are permanently attached to anarticle to be tracked and, once attached, the serial number of the tagis associated with its host article in a computer data base. Forexample, a particular type of medicine may be contained in hundreds orthousands of small vials. Upon manufacture, or receipt of the vials at ahealth care institution, an RFID tag is attached to each vial. Each vialwith its permanently attached RFID tag will be checked into the database of the health care institution upon receipt. The RFIDidentification number may be associated in the data base with the typeof medicine, size of the dose in the vial, and perhaps other informationsuch as the expiration date of the medicine. Thereafter, when the RFIDtag of a vial is interrogated and its identification number read, thedata base of the health care institution can match that identificationnumber with its stored data about the vial. The contents of the vial canthen be determined as well as any other characteristics that have beenstored in the data base. This system requires that the institutionmaintain a comprehensive data base regarding the articles in inventoryrather than incorporating such data into an RFID tag.

An object of the tag is to associate it with an article throughout thearticle's life in a particular facility, such as a manufacturingfacility, a transport vehicle, a health care facility, a storage area,or other, so that the article may be located, identified, and tracked,as it is moved. For example, knowing where certain medical articlesreside at all times in a health care facility can greatly facilitatelocating needed medical supplies when emergencies arise. Similarly,tracking the articles through the facility can assist in generating moreefficient dispensing and inventory control systems as well as improvingwork flow in a facility. Additionally, expiration dates can be monitoredand those articles that are older and about to expire can be moved tothe front of the line for immediate dispensing. This results in betterinventory control and lowered costs.

Other RFID tags are writable and information about the article to whichthe RFID tag is attached can be programmed into the individual tag.While this can provide a distinct advantage when a facility's computerservers are unavailable, such tags cost more, depending on the size ofthe memory in the tag. Programming each one of the tags with informatoncontained in the article to which they are attached involves furtherexpense.

RFID tags may be applied to containers or articles to be tracked by themanufacturer, the receiving party, or others. In some cases where amanufacturer applies the tags to the product, the manufacturer will alsosupply a respective data base file that links the identification numberof each of the tags to the contents of each respective article. Thatmanufacturer supplied data base can be distributed to the customer inthe form of a file that may easily be imported into the customer'soverall data base thereby saving the customer from the expense ofcreating the data base.

Many RFID tags used today are passive in that they do not have a batteryor other autonomous power supply and instead, must rely on theinterrogating energy provided by an RFID reader to provide power toactivate the tag. Passive RFID tags require an electromagnetic field ofenergy of a certain frequency range and certain minimum intensity inorder to achieve activation of the tag and transmission of its storeddata. Another choice is an active RFID tag; however, such tags requirean accompanying battery to provide power to activate the tag, thusincreasing the expense of the tag and making them undesirable for use ina large number of applications.

Depending on the requirements of the RFID tag application, such as thephysical size of the articles to be identified, their location, and theability to reach them easily, tags may need to be read from a shortdistance or a long distance by an RFID reader. Such distances may varyfrom a few centimeters to ten or more meters. Additionally, in the U.S.and in other countries, the frequency range within which such tags arepermitted to operate is limited. As an example, lower frequency bands,such as 125 KHz and 13.56 MHz, may be used for RFID tags in someapplications. At this frequency range, the electromagnetic energy isless affected by liquids and other dielectric materials, but suffersfrom the limitation of a short interrogating distance. At higherfrequency bands where RFID use is permitted, such as 915 MHz and 2.4GHz, the RFID tags can be interrogated at longer distances, but theyde-tune more rapidly as the material to which the tag is attachedvaries. It has also been found that at these higher frequencies, closelyspaced RFID tags will de-tune each other as the spacing between tags isdecreased.

There are a number of common situations where the RFID tags may belocated inside enclosures. Some of these enclosures may have entirely orpartially metal or metallized surfaces. Examples of enclosures includemetal enclosures (e.g., shipping containers), partial metal enclosures(e.g., vehicles such as airplanes, buses, trains, and ships that have ahousing made from a combination of metal and other materials), andnon-metal enclosures (e.g., warehouses and buildings made of wood).Examples of objects with RFID tags that may be located in theseenclosures include loose articles, packaged articles, parcels insidewarehouses, inventory items inside buildings, various goods insideretail stores, and various portable items (e.g., passengeridentification cards and tickets, baggage, cargo, individual life-savingequipment such as life jackets and masks) inside vehicles, etc.

The read range (i.e., the range of the interrogation and/or responsesignals) of RFID tags is limited. For example, some types of passiveRFID tags have a maximum range of about twelve meters, which may beattained only in ideal free space conditions with favorable antennaorientation. In a real situation, the observed tag range is often sixmeters or less. Therefore, some of the enclosures described above mayhave dimensions that far exceed the read range of an individual RFIDtag. Unless the RFID reader can be placed in close proximity to a targetRFID tag in such an enclosure, the tag will not be activated and read.Additionally, metal surfaces of the enclosures present a seriousobstacle for the RF signals that need to be exchanged between RFIDreaders and RFID tags, making RFID tags located behind those metalsurfaces difficult or impossible to detect.

In addition to the above, the detection range of the RFID systems istypically limited by signal strength to short ranges, frequently lessthan about thirty centimeters for 13.56 MHz systems. Therefore, portablereader units may need to be moved past a group of tagged items in orderto detect all the tagged items, particularly where the tagged items arestored in a space significantly greater than the detection range of astationary or fixed single reader antenna. Alternately, a large readerantenna with sufficient power and range to detect a larger number oftagged items may be used. However, such an antenna may be unwieldy andmay increase the range of the radiated power beyond allowable limits.Furthermore, these reader antennae are often located in stores or otherlocations where space is at a premium and it is expensive andinconvenient to use such large reader antennae. In another possiblesolution, multiple small antennae may be used but such a configurationmay be awkward to set up when space is at a premium and when wiring ispreferred or required to be hidden.

In the case of medical supplies and devices, it is desirable to developaccurate tracking, inventory control systems, and dispensing systems sothat RFID tagged devices and articles may be located quickly should theneed arise, and may be identified for other purposes, such as expirationdates. In the case of medical supply or dispensing cabinets used in ahealth care facility, a large number of medical devices and articles arelocated closely together, such as in a plurality of drawers. Cabinetssuch as these are typically made of metal, which can make the use of anexternal RFID system for identification of the stored articlesdifficult. In some cases, such cabinets are locked due to the presenceof narcotics or other medical articles or apparatus within them that aresubject to a high theft rate. Thus, manual identification of the cabinetcontents is difficult due to the need to control access.

Providing an internal RFID system in such a cabinet can pose challenges.Where internal articles can have random placement within the cabinet,the RFID system must be such that there are no “dead zones” that theRFID system is unable to reach. In general, dead zones are areas inwhich the level of coupling between an RFID reader antenna and an RFIDtag is not adequate for the system to perform a successful read of thetag. The existence of such dead zones may be caused by orientations inwhich the tag and the reader antennae are in orthogonal planes. Thus,articles placed in dead zones may not be detected thereby resulting ininaccurate tracking of tagged articles.

Often in the medical field, there is a need to read a large number oftags attached to articles in such an enclosure, and as mentioned above,such enclosures have limited access due to security reasons. Thephysical dimension of the enclosure may need to vary to accommodate alarge number of articles or articles of different sizes and shapes. Inorder to obtain an accurate identification and count of suchclosely-located medical articles or devices, a robust electromagneticenergy field must be provided at the appropriate frequency within theenclosure to surround all such stored articles and devices to be surethat their tags are all are activated and read. Such medical devices mayhave the RFID tags attached to the outside of their containers and maybe stored in various orientations with the RFID tag (and associatedantenna) pointed upwards, sideways, downward, or at some other angle ina random pattern.

Generating such a robust EM energy field is not an easy task. Where theenclosure has a size that is resonant at the frequency of operation, itcan be easier to generate a robust EM field since a resonant standingwave may be generated within the enclosure. However, in the RFID fieldthe usable frequencies of operation are strictly controlled and arelimited. It has been found that enclosures are desired for the storageof certain articles that do not have a resonant frequency that matchesone of the allowed RFID frequencies. Thus, a robust EM field must beestablished in another way.

Additionally, where EM energy is introduced to such an enclosure forreading the RFID tags within, efficient energy transfer is ofimportance. Under static conditions, the input or injection of EM energyinto an enclosure can be maximized with a simple impedance matchingcircuit positioned between the conductor delivering the energy and theenclosure. As is well known to those of skill in the art, such impedancematching circuits or devices maximize the power transfer to theenclosure while minimizing the reflections of power from the enclosure.Where the enclosure impedance changes due to the introduction or removalof articles to or from the enclosure, a static impedance matchingcircuit may not provide optimum energy transfer into the enclosure. Ifthe energy transfer and resulting RF field intensity within theenclosure were to fall below a threshold level, some or many of the tagson articles within the enclosure would not be activated to identifythemselves, leaving an ineffective inventory system.

It is a goal of many health care facilities to keep the use of EM energyto a minimum, or at least contained. The use of high-power readers tolocate and extract data from RFID tags is generally undesirable inhealth care facilities, although it may be acceptable in warehouses thatare sparsely populated with workers, or in aircraft cargo holds.Radiating a broad beam of EM energy at a large area, where that EMenergy may stray into adjacent, more sensitive areas, is undesirable.Efficiency in operating a reader to obtain the needed identificationinformation from tags is an objective. In many cases where RFID tags areread, hand-held readers are used. Such readers transmit a relativelywide beam of energy to reach all RFID tags in a particular location.While the end result of activating each tag and reading it may beaccomplished, the transmission of the energy is not controlled except bythe aim of the user. Additionally, this is a manual system that willrequire the services of one or more individuals, which can also beundesirable in facilities where staff is limited

Hence, those of skill in the art have recognized a need for an RFID tagreader system in which the efficient use of energy is made to activateand read all RFID tags in an enclosed area. A further need forestablishing a robust EM field in enclosures to activate and read tagsdisposed at random orientations has also been recognized. A further needhas been recognized for an automated system to identify articles storedin a metal cabinet without the need to gain access to the cabinet. Afurther need has been recognized for a dynamic matching circuit thatwill automatically adjust the matching circuit operation in dependenceon the changes to the enclosure impedance. The present inventionfulfills these needs and others.

SUMMARY OF THE INVENTION

Briefly and in general terms, the present invention is directed to asystem for reading a data carrier that is attached to an article, thedata carrier being responsive to electromagnetic energy (EM) of afrequency f₁ in response to which the data carrier provides uniqueidentification data, the system comprising a metallic enclosure havingelectrically conductive walls, the enclosure have a natural frequency ofresonance f₂, a probe mounted through one of the walls of the enclosure,the probe configured to inject electromagnetic energy into theenclosure, wherein the position of the probe in relation to the walls ofthe enclosure is selected to optimize power transfer into the enclosure,a storage device located within the enclosure configured to receive datacarriers with respective articles to be stored, and a receiving antennadisposed within the enclosure and configured to receive the uniqueidentification data provided by the data carriers.

In a further aspect in accordance with the invention, the frequency f₁is different from the natural frequency of resonance of the enclosuref₂, and the position of the probe is selected so that reflected phase ofEM from the walls is equal at the probe position, i.e, they are in phaseat the probe position.

In yet more detailed aspect, the system further comprises an impedancematching circuit configured to more closely match impedance of the probeto impedance of the enclosure. The impedance matching circuit comprisesan active impedance matching circuit.

In other aspects in accordance with the invention, the storage devicecomprises a drawer adapted to move into and out of the enclosure. Aportion of the drawer forms a portion of the electrically conductiveenclosure when the drawer is in a predetermined position in relation tothe enclosure. The drawer is non-electrically conductive except for aportion of the drawer that forms a portion of the electricallyconductive enclosure. Further, the drawer is slidable into and out ofthe enclosure through an opening in the enclosure, the drawer having afront panel that is electrically conductive and that contacts theelectrically conductive enclosure when the drawer is slid to apredetermined position within the enclosure.

In another more detailed aspect, the receiving antenna is located on anopposite side of the storage device from the probe.

In yet further aspects, the system further comprises a plurality ofprobes, each probe being mounted through a wall of the enclosure, eachof the probes configured to inject electromagnetic (EM) energy into theenclosure at its respective position, wherein the position of each probein relation to the walls of the enclosure is selected to optimize powertransfer into the enclosure to provide coverage of a portion of theenclosure with an electromagnetic field, the position of the probesfurther selected such that the probes may be sequentially activated toinject their respective EM energy into the enclosure to provide acomposite EM field.

In yet other detailed aspect, there is provided a system for tracking anarticle having a data carrier attached thereto, the data carrier beingresponsive to electromagnetic energy of a frequency f₁ in response towhich the data carrier broadcasts unique identification data, the systemcomprising a metallic enclosure having electrically conductive walls,the enclosure have a natural frequency of resonance of f₂, wherein f₂ isdifferent from f₁, a probe mounted through one of the walls of theenclosure, the probe configured to inject electromagnetic energy intothe enclosure, wherein the position of the probe in relation to thewalls of the enclosure is selected to optimize power transfer into theenclosure, a drawer movably disposed within the enclosure, the drawerconfigured to store articles having respective data carriers, and areceiving antenna disposed within the enclosure and configured toreceive the unique identification data provided by data carriers.

In yet another aspect in accordance with the invention, there isprovided a system for establishing a robust electromagnetic field withinan enclosure, the system comprising a metallic enclosure havingelectrically conductive walls, the enclosure have a natural frequency ofresonance f₂, a probe mounted through one of the walls of the enclosure,the probe configured to inject electromagnetic energy (EM) into theenclosure, the EM having a frequency of f₁, wherein f₁ and f₂ aredifferent, wherein the position of the probe in relation to the walls ofthe enclosure is selected to optimize power transfer into the enclosure,a storage device located within the enclosure configured to receive datacarriers with respective articles to be stored, and a receiving antennadisposed within the enclosure and configured to receive the uniqueidentification data provided by the data carriers.

The features and advantages of the invention will be more readilyunderstood from the following detailed description that should be readin conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a drawer that may be positioned withina medical dispensing cabinet, showing the storage of a plurality ofmedical articles randomly positioned in the drawer, each of thosearticles having an integral RFID tag oriented randomly;

FIG. 2 is a perspective view of a medication dispensing cabinet havingfive drawers, one of which is similar to the schematic view of FIG. 1,the cabinet also having an integral computer for controlling access tothe cabinet and performing inventory tracking by periodically readingany RFID tags placed on articles stored within the cabinet, and forreporting the identified articles to a remote computer;

FIG. 3 is a block and flow diagram showing an embodiment in which anRFID reader transmits activating EM energy into a drawer containing RFIDtags with a single transmitting antenna, receives the data output fromthe activated RFID tags with a single receiving antenna, a computercontrolling the transmission of activating energy and receiving the datafrom the activated RFID tags for processing;

FIG. 4 is a block and flow diagram similar to FIG. 3 showing anembodiment in which an RFID reader transmits activating EM energy into adrawer containing RFID tags with two transmitting antennae, receives thedata output from the activated RFID tags with three receiving antennae,and as in FIG. 3, a computer controlling the transmission of activatingenergy and receiving the data from the activated RFID tags forprocessing;

FIG. 5 shows an enclosure with a single probe and a connector, the probebeing configured to inject EM energy into the enclosure and excite a TEmode;

FIG. 6 shows an enclosure with a single probe and a connector, the probebeing configured to inject EM energy into the enclosure and excite a TMmode;

FIG. 7 shows a plot of coupled power in an enclosure as a function offrequency for a resonant enclosure where F_(n) is the natural resonancefrequency of the enclosure;

FIG. 8 shows a plot of coupled power (ordinate axis) in an enclosure asa function of frequency (abscissa axis), where f_(f) is a forcedresonance frequency, or otherwise referred to as a frequency that is notequal to the resonant frequency of the enclosure, and f_(n) is thenatural resonant frequency of the enclosure, showing the establishmentof a robust field of coupled power in the enclosure at the f_(f)frequency;

FIG. 9 shows an enclosure with two probes each with a connector forinjecting EM energy into the enclosure, one probe being a TM probe andthe other being a TE probe;

FIG. 10 shows a probe, a connector, and an attenuator that is used toimprove the impedance match between the probe and the enclosure;

FIG. 11 shows a probe, a connector, and a passive matching circuit thatis used to improve the impedance match between the probe and enclosure;

FIG. 12 shows an active matching circuit connected between a probelocated in an enclosure and a transceiver, the active matching circuitcomprising a tunable capacitor, a dual-directional coupler, multiplepower sensors, and a comparator used to provide a closed-loop, variablematching circuit to improve the impedance match between the probe andthe enclosure;

FIG. 13 provides a side cross-sectional view of the cabinet of FIG. 2 atthe location of a drawer with the drawer removed for clarity, showingthe placement of two probe antennae in a “ceiling mount” configurationfor establishing a robust EM field in the drawer when it is in place inthe cabinet in the closed position;

FIG. 14 is a perspective view of the metallic enclosure showing theprobe configuration of FIG. 13 again showing the two probe antennae forestablishing a robust EM field in a drawer to be inserted;

FIG. 15 is a cutaway perspective side view of the metallic enclosure orframe in which are mounted the dual probe antennae of FIGS. 13 and 14with the drawer removed for clarity;

FIG. 16 is a frontal perspective view of the view of FIG. 14 with acutaway plastic drawer in place in the metallic enclosure and furthershowing the dual ceiling mount probe antennae protected by anelectromagnetically inert protective cover, and further showing coolingsystem components mounted at the back of the cabinet near the drawer'sback, the drawing also showing a partial view of a drawer slidemechanism for ease in sliding the drawer between open and closedpositions in the cabinet, the drawer front and rear panels having beencutaway in this view;

FIG. 17 is a frontal perspective view at the opposite angle from that ofFIG. 16 with the plastic drawer completely removed showing the dualceiling mount probe antennae protected by the EM inert protective covermounted to the metallic enclosure, and further showing the coolingsystem components of FIG. 16 mounted at the back of the cabinet as aspring loading feature to automatically push the drawer to the openposition when the drawer's latch is released, the figure also showing amounting rail for receiving the slide of the drawer;

FIG. 18 is a schematic view with measurements in inches of the placementof two TE₀₁ mode probes in the top surface of the enclosure shown inFIGS. 13-15;

FIG. 19 is a schematic view of the size and placement within the drawerof FIG. 16 of two microstrip or “patch” antennae and their microstripconductors disposed between respective antennae and the back of thedrawer at which they will be connected to SMA connectors in oneembodiment, for interconnection with other components;

FIG. 20 is diagram of field strength in an embodiment of an enclosurewith a probe placed in the enclosure at a position in accordance withthe diagram of FIG. 19;

FIG. 21 is a lower scale drawing of the field intensity diagram of FIG.20 showing a clearer view of the field intensity nearer the front andback walls of the enclosure; and

FIGS. 22A and 22B together present a block electrical and signal diagramfor a multiple-drawer medical cabinet, such as that shown in FIG. 2,showing the individual multiplexer switches, the single RFID scanner,and power control.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now in more detail to the exemplary drawings for purposes ofillustrating embodiments of the invention, wherein like referencenumerals designate corresponding or like elements among the severalviews, there is shown in FIG. 1 a schematic representation of a partialenclosure 20 in which a plurality of medical articles 22 are stored,each with a respective RFID tag 24 that has a unique identificationnumber. The partial enclosure may comprise a drawer having a front 26, aleft side 28, a right side 30, a rear 32, and a bottom 34. Thesearticles are randomly distributed in the drawer with the RFID tagsfacing in various and random directions.

As used in regard to the embodiments herein, “reader” and “interrogator”refer to a device that may read or write/read. The data capture deviceis always referred to as a reader or an interrogator regardless ofwhether it can only read or is also capable of writing. A readertypically contains a radio frequency module (a transmitter and areceiver, sometimes referred to as a “transceiver”), a control unit anda coupling element (such as an antenna or antennae) to the RFID tag.Additionally, many readers include an interface for forwarding dataelsewhere, such as an RS-232 interface. The reader, when transmitting,has an interrogation zone within which an RFID tag will be activated.When within the interrogation zone, the RFID tag will draw its powerfrom the electrical/magnetic field created in the interrogation zone bythe reader. In a sequential RFID system (SEQ), the interrogation fieldis switched off at regular intervals. The RFID tag is programmed torecognize these “off” gaps and they are used by the tag to send data,such as the tag's unique identification number. In some systems, thetag's data record contains a unique serial number that is incorporatedwhen the tag is manufactured and which cannot be changed. This numbermay be associated in a data base with a particular article when the tagis attached to that article. Thus, determining the location of the tagwill then result in determining the location of the article to which itis attached. In other systems, the RFID tag may contain more informationabout the article to which it is attached, such as the name oridentification of the article, its expiration date, it dose, the patientname, and other information. The RFID tag may also be writable so thatit can be updated.

As used in regard to the embodiments herein, “tag” is meant to refer toan RFID transponder. Such tags typically have a coupling element, suchas an antenna, and an electronic microchip. The microchip includes datastorage, also referred to as memory.

FIG. 2 presents a representative medical dispensing cabinet 40comprising a plurality of movable drawers 42. In this embodiment, thereare five drawers that slide outwardly from the cabinet so that access isprovided to the contents of the drawers. FIG. 1 is a schematic diagramof a representative drawer that may be positioned within the cabinet ofFIG. 2 for sliding outward to provide access to the drawer's contentsand for sliding inward into the cabinet to secure the drawer's contents.The cabinet also comprises an integral computer 44 that may be used tocontrol access to the drawers and to generate data concerning access andcontents, and to communicate with other systems. In this embodiment, thecomputer generates data concerning the number and type of articles inthe drawers, the names of the patients for whom they have beenprescribed, the prescribed medications and their prescribedadministration dates and times, as well as other information. In asimpler system, the computer may simply receive unique identificationnumbers from stored articles and pass those identification numbers to aninventory control computer that has access to a data base for matchingthe identification numbers to article descriptions.

Such a cabinet may be located at a nursing station on a particular floorof a health care institution and may contain the prescriptions for thepatients of that floor. As prescriptions are prepared for the patientsof that floor, they are delivered and placed into the cabinet 40. Theyare logged into the integral computer 44, which may notify the pharmacyof their receipt. A drawer may also contain non-prescription medicalsupplies or articles for dispensing to the patients as determined by thenursing staff. At the appropriate time, a nurse would access the drawerin which the medical articles are stored through the use of the computer44, remove a particular patient's prescriptions and any needednon-prescription articles, and then close the drawer so that it issecured. In order to access the cabinet, the nurse may need to providevarious information and may need a secure access code. The drawers 42may be locked or unlocked as conditions require.

The computer 44 in some cases may be in communication with otherfacilities of the institution. For example, the computer 44 may notifythe pharmacy of the health care institution that a patient'sprescription has been removed from the cabinet for administration at aparticular day and time. The computer may also notify the financedepartment of the health care institution of the removal ofprescriptions and other medical articles for administration to aparticular patient. This medication may then be applied to the patient'saccount. Further, the computer 44 may communicate to administration forthe purpose of updating a patient's Medication Administration Record(MAR), or e-MAR. The medication cabinet 40 computer 44 may be wirelesslyconnected to other computers of the health care institution or may havea wired connection. The cabinet may be mounted on wheels and may bemoved about as needed or may be stationary and unable to move.

Systems that use RFID tags often employ an RFID reader in communicationwith one or more host computing systems that act as depositories tostore, process, and share data collected by the RFID reader. Turning nowto FIGS. 3 and 4, a system and method 50 for tracking articles are shownin which a drawer 20 of the cabinet 40 of FIG. 2 is monitored to obtaindata from RFID tags disposed with articles in that drawer. As mentionedabove, a robust field of EM energy needs to be established in thestorage site so that the RFID tags mounted to the various storedarticles will be activated, regardless of their orientation.

In FIGS. 3 and 4, the tracking system 50 is shown for identifyingarticles in an enclosure and comprises a transmitter 52 of EM energy aspart of an RFID reader. The transmitter 52 has a particular frequency,such as 915 MHz, for transmitting EM energy into a drawer 20 by means ofa transmitting antenna 54. The transmitter 52 is configured to transmitthe necessary RFID EM energy and any necessary timing pulses and datainto the enclosure 20 in which the RFID tags are disposed. In this case,the enclosure is a drawer 20. The computer 44 of an RFID reader 51controls the EM transmitter 52 to cycle between a transmit period and anon-transmit, or off, period. During the transmit period, thetransmitted EM energy at or above a threshold intensity level surroundsthe RFID tags in the drawer thereby activating them. The transmitter 52is then switched to the off period during which the RFID tags respondwith their respective stored data.

The embodiment of FIG. 3 comprises a single transmitting probe antenna54 and a single receiving antenna 56 oriented in such a manner so as tooptimally read the data transmitted by the activated RFID tags locatedinside the drawer 20. The single receiving antenna 56 is communicativelycoupled to the computer 44 of the reader 50 located on the outside ofthe drawer 20 or on the inner bottom of the drawer. Other mountinglocations are possible. Coaxial cables 58 or other suitable signal linkscan be used to couple the receiving antenna 56 to the computer 44. Awireless link may be used in a different embodiment. Although not shownin the figures, those skilled in the art will recognize that variousadditional circuits and devices are used to separate the digital datafrom the RF energy, for use by the computer. Such circuits and deviceshave not been shown in FIGS. 3 and 4 to avoid unneeded complexity in thedrawing.

The embodiment of FIG. 4 is similar to the embodiment of FIG. 3 butinstead uses two transmitting probe antennae 60 and 62 and threereceiving antennae 64, 66, and 68. The configuration and the number oftransmitting probe antennae and receiving antennae to be used for asystem may vary based at least in part on the size of the enclosure 20,the frequency of operation, the relationship between the operationfrequency and the natural resonance frequency of the enclosure, and theexpected number of RFID tags to be placed in it, so that all of the RFIDtags inside the enclosure can be reliably activated and read. Thelocation and number of RFID reader components can be dependent on theparticular application. For example, fewer components may be requiredfor enclosures having a relatively small size, while additionalcomponents, such as shown in FIG. 4, may be needed for largerenclosures. Although shown in block form in FIGS. 3 and 4, it should berecognized that each receiving antenna 56, 64, 66, and 68 of the system50 may comprise a sub-array in a different embodiment.

The transmit antennae (54, 60, and 62) and the receive antennae (56, 64,66, and 68) may take different forms. In one embodiment as is discussedin more detail below, a plurality of “patch” or microstrip antennae wereused as the reader receiving antennae and were located at positionsadjacent various portions of the bottom of the drawer while the transmitantennae were wire probes located at positions adjacent portions of thetop of the drawer. It should be noted that in the embodiments of FIGS. 3and 4, the RFID reader 50 may be permanently mounted in the same cabinetat a strategic position in relation to the drawer 20.

One solution for reliably interrogating densely packed or randomlyoriented RFID tags in an enclosure is to treat the enclosure as aresonant cavity. Establishing a resonance within the cavity enclosurecan result in a robust electromagnetic field capable of activating allRFID tags in the enclosure. This can be performed by building anenclosure out of electrically conductive walls and exciting the metallicenclosure, or cavity, using a probe or probes to excite transverseelectric (TE) or transverse magnetic (TM) fields in the cavity at thenatural frequency of resonance of the cavity. This technique will workif the cavity dimensions can be specifically chosen to set up theresonance at the frequency of operation or if the frequency of operationcan be chosen for the specific enclosure size. Since there are limitedfrequency bands available for use in RFID applications, varying the RFIDfrequency is not an option for many applications. Conversely, requiringa specific set of physical dimensions for the enclosure so that thenatural resonant frequency of the enclosure will equal the availableRFID tag activating frequency will restrict the use of this techniquefor applications where the enclosure needs to be of a specific size.This latter approach is not practical in view of the many differentsizes, shapes, and quantities of medical articles that must be stored.

Referring now to FIG. 5, a rectangular enclosure 80 is provided that maybe formed as part of a medical cabinet, such as the cabinet shown inFIG. 2. It may be embodied as a frame disposed about a non-metallicdrawer in such a cabinet. The enclosure 80 is formed of metallic ormetallized walls 82, floor 83, and ceiling 84 surfaces, all of which areelectrically conductive. All of the walls 82, floor 83, and ceiling 84may also be referred to herein as “walls” of the enclosure. FIG. 5 alsoshows the use of an energy coupling or probe 86 located at he topsurface 84 of the enclosure 80. In this embodiment, the probe takes theform of a capacitor probe 88 in that the probe 88 has a first portion 94that proceeds axially through a hole 90 in the ceiling 84 of theenclosure. The purpose of the coupling is to efficiently transfer theenergy from the source 52 (see FIGS. 3 and 4) to the interior 96 of theenclosure 80. The size and the position of the probe are selected foreffective coupling and the probe is placed in a region of maximum fieldintensity. In FIG. 5, a TE₀₁ mode is established through the use ofcapacitive coupling. The length and distance of the bent portion 94 ofthe probe 88 affects the potential difference between the probe and theenclosure 80.

Similarly, FIG. 6 presents an inductive coupling 110 of the externalenergy to an enclosure 112. The coupling takes the form of a loop probe114 mounted through a side wall 116 of the enclosure. The purpose ofthis probe is to establish a TM₀₁ mode in the enclosure.

The rectangular enclosures 80 and 112 shown in FIGS. 5 and 6 each have anatural frequency of resonance f_(n), shown in FIG. 7 and indicated onthe abscissa axis 118 of the graph by f_(n). This is the frequency atwhich the coupled power in the enclosure is the highest, as shown on theordinate axis 119 of the graph. If the injected energy to the enclosuredoes not match the f_(n) frequency, the coupled power will not benefitfrom the resonance phenomenon of the enclosure. In cases where thefrequency of operation cannot be changed, and is other than f_(n), andthe size of the enclosure cannot be changed to obtain an f_(n) that isequal to the operating frequency, another power coupling apparatus andmethod must be used. In accordance with aspects of the invention, anapparatus and method are provided to result in a forced resonance f_(f)within the enclosure to obtain a standing wave within the enclosure withconstructive interference. Such a standing wave will establish a robustenergy field within the enclosure strong enough to activate all RFIDtags residing therein.

When an EM wave that is resonant with the enclosure enters, it bouncesback and forth within the enclosure with low loss. As more wave energyenters the enclosure, it combines with and reinforces the standing wave,increasing its intensity (constructive interference). Resonation occursat a specific frequency because the dimensions of the cavity are anintegral multiple of the wavelength at the resonance frequency. In thepresent case where the injected energy is not at the natural resonancefrequency f_(n) of the enclosure, a solution in accordance with aspectsof the invention is to set up a “forced resonance” in an enclosure. Thisforced resonance is different from the natural resonance of theenclosure in that the physical dimensions of the enclosure are not equalto an integral multiple of the wavelength of the excitation energy, asis the case with a resonant cavity. A forced resonance can be achievedby determining a probe position, along with the probe length to allowfor energy to be injected into the cavity such that constructiveinterference results and a standing wave is established. The energyinjected into the enclosure in this case will set up an oscillatoryfield region within the cavity, but will be different from a standingwave that would be present at the natural resonance frequency f_(n) of aresonant cavity. The EM field excited from this forced resonance will bedifferent than the field structure found at the natural resonance of aresonant cavity, but with proper probe placement of a probe, a robust EMfield can nevertheless be established in an enclosure for RFID taginterrogation. Such is shown in FIG. 8 where it will be noted that thecurve for the forced resonance f_(f) coupled power is close to that ofthe natural resonance f_(n).

Turning now to FIG. 9, an enclosure 120 having two energy injectionprobes is provided. The first probe 86 is capacitively coupled to theenclosure 120 in accordance with FIG. 5 to establish a TE₀₁ mode. Thesecond probe 114 is inductively coupled to the enclosure 120 inaccordance with FIG. 6 to establish a TM₀₁ mode. These two probes areboth coupled to the enclosure to inject energy at a frequency f_(f) thatis other than the natural resonance frequency f_(n) of the enclosure.The placement of these probes in relation to the ceiling 126 and walls128 of the enclosure will result in a forced resonance within theenclosure 120 that optimally couples the energy to the enclosure andestablishes a robust EM field within the enclosure for reading RFID tagsthat may be located therein. The placement of these probes in relationto the walls of the enclosure, in accordance with aspects of theinvention, result in the forced resonance curve f_(f) shown in FIG. 8.

Referring briefly to FIG. 10, an impedance matching circuit 121 is shownthat functions to match the impedance of a source of energy 122 to theenclosure 120. The impedance matching circuit is located between thecoaxial cable 122 that feeds activating energy to the enclosure 120 andthe capacitively coupled probe 88 through a hole in the metallic ceiling126 of the enclosure. While the hole is not shown in the drawing of FIG.10, the insulator 123 that electrically insulates the probe from themetallic ceiling is shown. In this case, the matching circuit 121consists of only a resistive attenuator 124 used to reduce reflectionsof energy by the enclosure 120. However, as will be appreciated by thoseof skill in the art, capacitive and inductive components are likely toexist in the enclosure and in the coupling 88. FIG. 11 on the other handpresents an impedance matching circuit 124 having passive reactivecomponents for use in matching the impedance of the coaxial cable/energysource 122 and the enclosure 120. In this exemplary impedance matchingcircuit 124, an inductive component 125 and a capacitive component 127are connected in series, although other configurations, including theaddition of a resistive component and other connection configurations,are possible.

Passive components such as resistors, inductors, and capacitors shown inFIGS. 10 and 11 can be used to form matching circuits to match theimpedances of the energy source and the enclosure. This will aid incoupling power into the enclosure. However, the passive matching circuitwill improve the impedance match for a specific enclosure loading, suchas an empty enclosure, partially loaded, or fully loaded enclosure. Butas the enclosure contents are varied, the impedance match may not beoptimized due to the variation in contents in the enclosure causing theimpedance properties of the enclosure to change.

This non-optimal impedance match caused by variation in enclosureloading can be overcome by the use of an active impedance matchingcircuit which utilizes a closed loop sensing circuit to monitor forwardand reflected power. Referring now to FIG. 12, an active matchingcircuit 130 is provided that comprises one or several fixed valuepassive components such as inductors 132, capacitors 134, or resistors(not shown). In addition, one or several variable reactance devices,such as a tunable capacitor 134, are incorporated into the circuit;these tunable devices making this an active impedance matching circuit.The tunable capacitor 134 can take the form of a varactor diode,switched capacitor assembly, MEMS capacitor, or BST (Barium StrontiumTitanate) capacitor. A control voltage is applied to the tunablecapacitor 134 and varied to vary the capacitance provide by the device.The tunable capacitor 134 provides the capability to actively change theimpedance match between the probe 140 and the enclosure 142.

To complete the active matching circuit, a dual directional coupler 144along with two power sensors 146 can be incorporated. The dualdirectional coupler 144 and the power sensors 146 provide the ability tosense forward and reflected power between the RFID transceiver 148 andthe active matching circuit 130 and enclosure 142. Continuous monitoringof the ratio of forward and reflected power by a comparator 150 providesa metric to use to adjust the tunable capacitor 134 to keep the probe140 impedance matched to the enclosure 142. An ability to continuouslymonitor and improve the impedance match as the contents of the enclosureare varied is provided with the active matching circuit 130.

Referring now to the side cross-sectional view of FIG. 13, twoceiling-mounted 160 probe antennae 162 and 164 are shown mounted withinan enclosure, which may also be referred to herein as a cavity 166,which in this embodiment, operates as a Faraday cage. As shown, theFaraday cage 166 comprises walls (one of which is shown) 168, a back170, a floor 172, a ceiling 160, and a front 161 (only the position ofthe front wall is shown). All surfaces forming the cavity areelectrically conductive, are electrically connected with one another,and are structurally formed to be able to conduct the frequency ofenergy f_(f) injected by the two probes 162 and 164. In this embodiment,the cavity 166 is constructed as a metal frame 167 that may form a partof a medical supply cabinet similar to that shown in FIG. 2. Into thatmetal frame may be mounted a slidable drawer. The slidable drawer inthis embodiment is formed of electrically inert material, that is, it isnot electrically conductive, except for the front. When the drawer isslid into the cabinet to a closed configuration, the electricallyconductive front panel of the drawer comes into electrical contact withanother part or parts of the metallic frame 167 thereby forming thefront wall 161 of the Faraday cage 167.

The amount of penetration or retention into the cavity by the centralconductor 180 of each probe is selected so as to achieve optimumcoupling. The length of the bent portion 94 of the probe is selected toresult in better impedance matching. The position of the probe inrelation to the walls of the cavity is selected to create a standingwave in the cavity. In this embodiment, the probe antennae 162 and 164have been located at a particular distance D1 and D3 from respectivefront 161 and back 170 walls. These probe antennae, in accordance withone aspect of the invention, are only activated sequentially after theother probe has become inactivated. It has been found that thisconfiguration results in a standing wave where the injected energy wavesare in phase so that constructive interference results.

FIG. 14 is a front perspective view of the probe configuration of FIG.13 again showing the two probe antennae 162 and 164 located in aFaraday-type enclosure 166 for establishing a robust EM field in anarticle storage drawer to be inserted. It should be noted again that theFaraday cavity 166 is constructed as a metallic frame 167. In thisfigure, the cavity is incomplete in that the front surface of the “cage”is missing. In one embodiment, this front surface is provided by anelectrically conductive front panel of a slidable drawer. When thedrawer is slid into the cabinet, the front panel will make electricalcontact with the other portions of the metallic frame 167 therebycompleting the Faraday cage 166, although other portions of the drawerare plastic or are otherwise non-electrically conductive. In theembodiment discussed and shown herein, the two probe antennae 162 and164 are both located along a centerline between the side walls 166 and168 of the frame 166. The enclosure in one embodiment was 19.2 incheswide with the probe antennae spaced 9.6 inches from each side wall. Thiscentered location between the two side walls was for convenience in thecase of one embodiment. The probes may be placed elsewhere in anotherembodiment. In this embodiment, the spacing of the probes 162 and 164from each other is of little significance since they are sequentiallyactivated. Although not shown, two receiving antennae will also beplaced into the Faraday cage 166 to receive response signals from theactivated RFID tags residing within the cavity 166.

It will also be noted from reference to the figures that the probes eachhave a bent portion used for capacitive coupling with the ceiling 160 ofthe cavity, as is shown in FIG. 13. The front probe 162 is bent forwardwhile the back probe 164 is bent rearward A purpose for thisconfiguration was to obtain more spatial diversity and obtain bettercoverage by the EM field established in the drawer. Other arrangementsmay be possible to achieve a robust field within the cavity 166.Additionally two probes were used in the particular enclosure 166 sothat better EM field coverage of the enclosure 166 would result.

FIG. 15 is a cutaway perspective side view of the dual probe antennae162 and 164 of FIGS. 13 and 14, also with the drawer removed forclarity. The front probe 162 is spaced from the left side wall by ½λ ofthe operating frequency F_(f) as shown. It will be noted that the probeseach have a bent portion used for capacitive coupling with the ceiling160 of the enclosure 166 as shown in FIG. 13. The front probe 162 isbent forward for coupling with the more forward portion of the enclosurewhile the back probe 164 is bent rearward for coupling with the morerearward portion of the enclosure 166 to obtain more spatial diversityand obtain better coverage by the EM field in the drawer. Otherarrangements may be possible to achieve a robust field and furtherspatial diversity and coverage within the enclosure.

FIG. 16 is a frontal upward-looking perspective view of the frame 167forming a Faraday cage 166 showing a portion of a drawer 180 that hasbeen slidably mounted within the frame 167. The front metallic panel ofthe drawer has been removed so that its sliding operation can be moreclearly seen. It will also be noted that the dual ceiling mount probeantennae 162 and 164 have been covered and protected by anelectromagnetically inert protective cover 182. The drawer is formed ofa non-metallic material, such as a plastic or other electromagneticinert material having a low RF constant. The back 184 of the drawer hasalso been cut away so that a cooling system comprising coils 186 and afan 188 located in the back of the frame 167 can be seen. In this case,the drawer 180 is slidably mounted to the Faraday cage frame withmetallic sliding hardware 190. The sliding hardware of the drawer is sonear the side of the frame 167 of the enclosure 166 and may be inelectrical contact with the metallic slide hardware of the side walls168 of the enclosure that these metallic rails will have only a smalleffect on the EM field established within the enclosure.

FIG. 17 is an upward looking, frontal perspective view at the oppositeangle from that of FIG. 16; however, the drawer has been removed. Theframe 167 in this embodiment includes a mounting rail 192 for receivingthe slide of the drawer 180. In this embodiment, the mounting rail isformed of a metallic material; however, it is firmly attached to a side168 of the Faraday cage and thus is in electrical continuity with thecage. The figure also shows a spring mechanism 194 used to assist insliding the drawer outward so that access to the articles stored in thedrawer may be gained. The spring is configured to automatically push thedrawer outward when the drawer's latch is released.

FIG. 18 is a schematic view showing measurements of the placement of twoTE₀₁ mode capacitive coupling probes 162 and 164 in the ceiling 160 ofthe frame 167 shown in FIGS. 13-15. In this embodiment, the frequency ofoperation with the RFID tags is 915 MHz, which therefore has awavelength of 0.32764 meters or 1.07494 feet. One-half wavelength istherefore 0.16382 meters or 6.4495 inches. The length of the capacitivecoupling bent portion 200 of each of the probes is 5.08 cm or 2.00 in.The length of the axial extension 202 of the probes into the enclosureis 3.81 cm or 1.50 in., as measured from the insulator 204 into theenclosure 166. The probe configuration and placement in the embodimentwas based on an operation frequency of 915 MHz. In one embodiment, theenclosure 166 had a depth of 16.1 inches (40.89 cm), a width of 19.2inches (48.77 cm) and a height of 3 inches (7.62 cm). It was found thatthe optimum probe placements for this size and shape (rectangular)enclosure and for the 915 MHz operating frequency were: the front probewas spaced from the front wall by 5.0 inches (12.7 cm) and the rearprobe was spaced from the back wall by 5.0 inches (12.7 cm). As discussabove, the probes in this embodiment would only be activatedsequentially.

FIG. 19 is a schematic view of the size and placement within theenclosure 166 of FIG. 16 of two microstrip or “patch” antennae 210 and212 and their microstrip conductors 214 and 216 disposed between therespective antennae and the back of the enclosure at which they will beconnected to SMA connectors (not shown) in one embodiment. Feed lines 58(FIG. 3) may be connected to those SMA connectors and routed to thecomputer 44 for use in communicating the RFID signals for furtherprocessing. The measurements of the spacing of some of the microstripcomponents are provided in inches. The spacing of 9.7 in. is equivalentto 24.64 cm. The width of the microstrip line of 0.67 in. is equivalentto 17.0 mm. The spacing of 1.4 in. is equivalent to 3.56 cm. Otherconfigurations and types of receiving antennae may be used, as well asdifferent numbers of such antennae. In the present embodiment, thereceiving antennae are mounted on insulation at the bottom insidesurface of the metallic enclosure frame 167 so that the receiving patchantennae are not in contact with the metal surfaces of the Faraday cage.

Referring now to FIG. 20, the field intensity or field strength in theenclosure discussed above is shown with the ordinate axis shown involts/meter and the abscissa axis shown in meters. It will be seen fromthe diagram that the maximum field intensity occurs at about 5.0 inches(0.127 m) which results from the probe positioned at 5.0 inches (12.7cm) from the front wall and at a 915 MHz operating frequency. Referringnow to FIG. 21, the scale has been reduced although the large rise infield intensity can be seen at 5.0 inches. It can also be more clearlyseen that the field intensity falls off at the right wall but remainsstrong very close to the left wall. Therefore in an embodiment, a secondprobe was used that was placed 5.0 inches (12.7 cm) from the right wallthereby resulting in a mirror image field intensity to that shown inFIG. 21. The two probes 162 and 164 are activated sequentially and arenot both activated simultaneously. It will be noted that better EM fieldcoverage of the enclosure 166 is obtained with the two probes and thatRFID tags on articles positioned close to the front wall 161 will beactivated by the front probe 162 and that RFID tags on articlespositioned close to the rear wall 170 will be activated by the rearprobe 164 (see FIG. 13).

Although not intending to be bound by theory, in deriving the probelocation for TE modes in a square or rectangular non-resonant cavity,the following equation can be useful:

$N = {2 \times \frac{L_{2} - L_{1}}{\lambda_{g}}}$

where:

-   -   N=positive non-zero integer, for example 1, 2, 3, etc.    -   L₁=distance between probe and back wall    -   L₂=distance between probe and front wall    -   λ_(g)=wavelength in the cavity

L₁ cannot be zero for TE modes, which implies that the probe for TE modeexcitation cannot be at the front or back wall. For TM modes, theequation is the same, but N can equal zero as well as other positiveintegers. The probe position cannot be λ_(g)/2 from the front or backwall. An L₁ and an L₂ are chosen such that N can be a positive integerthat satisfies the equation. For example, for the enclosure 166discussed above:

L₁=4.785 inches

L₂=11.225 inches

λ_(g)=12.83 inches

Therefore,

$N = {{2 \times \frac{11.215 - 4.785`}{12.83}} = 1.0}$

The actual enclosure had the probe located at a slightly differentlocation (5.0 inches) than that indicated by the equation (4.785 inches)which was possibly due to the insertion of a plastic drawer in thecavity, which introduces a change in the phase from the reflectedsignals. The equation above is set up such that the reflected phase fromboth front and back walls is equal, i.e., they are “in phase” at theprobe location.

The wavelength in the enclosure, λ_(g), can be calculated usingwaveguide equations. Equations for a rectangular cavity are shown below.The cutoff frequency is required for this calculation. The equationswill change for a cylindrical cavity or for other shapes.

The cutoff frequency is at the point where g vanishes. Therefore, thecutoff frequency in Hertz is:

$( f_{c} )_{mn} = {\frac{1}{2\pi \sqrt{\mu ɛ}}\sqrt{( \frac{m\; \pi}{a} )^{2} + ( \frac{n\; \pi}{b} )^{2}}({Hz})}$

The cutoff wavelength in meters is:

$( \lambda_{c} )_{mn} = {\frac{2}{\sqrt{( \frac{m\;}{a} )^{2} + ( \frac{n\;}{b} )^{2}}}(m)}$

where:

-   -   a=inside width    -   b=inside height    -   m=number of ½-wavelength variations of fields in the “a”        direction    -   n=number of ½-wavelength variations of fields in the “b”        direction    -   ξ=permittivity    -   μ=permeability

The mode with the lowest cutoff frequency is called the dominant mode.Since TE₁₀ mode is the minimum possible mode that gives nonzero fieldexpressions for rectangular waveguides, it is the dominant mode of arectangular waveguide with a>b and so the dominant frequency is:

$( f_{c} )_{10} = {\frac{1}{2a\sqrt{\mu ɛ}}({Hz})}$

The wave impedance is defined as the ratio of the transverse electricand magnetic fields. Therefore, impedance is:

$Z_{TE} = {\frac{E_{x}}{H_{y}} = {\frac{j\; w\; \mu}{\gamma} = { \frac{j\; w\; \mu}{j\beta}\Rightarrow Z_{TE}  = \frac{k\; \eta}{\beta}}}}$

The guide wavelength is defined as the distance between two equal phaseplanes along the waveguide and it is equal to:

$\lambda_{g} = {{\frac{2\pi}{\beta} > \frac{2\pi}{k}} = \lambda}$${{{where}\mspace{14mu} k_{c}} = \sqrt{( \frac{{m\; \pi}\;}{a} )^{2} + ( \frac{{n\; \pi}\;}{b} )^{2}}};{and}$$\beta = \sqrt{k^{2} - k_{c}^{2}}$

FIGS. 22A and 22B together provide a block electrical and signal diagramfor a multiple-drawer medical cabinet, such as that shown in FIG. 2. Inthis case, the cabinet has eight drawers 220, shown in both FIGS. 22Aand 22B. Each drawer includes two top antennae, two bottom antennae anda lock with a lock sensor 222 for securing the drawer. Signals to andfrom the antennae of each drawer are fed through an RF multiplexerswitch 224. Each RF multiplexer switch 224 in this embodiment handlesthe routing of RF signals for two drawers. RFID activation field andRFID received signals are fed through the respective RF multiplexerswitch 224 to a main RFID scanner 230 (see FIG. 22B). The scanner 230output is directed to a microprocessor 232 (see FIG. 22B) for use incommunicating relevant information to remote locations, in this case bywired connection 234 and wireless connection 236 (see FIG. 22B). Varioussupport systems are also shown in FIGS. 22A and 22B, such as powerconnections, power distribution, back up battery (see FIG. 22B),interconnection PCBA, USB support (see FIG. 22A), cooling (see FIG.22B), and others.

In accordance with one embodiment, drawers are sequentially monitored.Within each drawer, the antennae are sequentially activated by theassociated multiplexer 224. Other embodiments for the signal andelectrical control systems are possible.

Although RFID tags are used herein as an embodiment, other data carriersthat communicate through electromagnetic energy may also be usable.

Unless the context requires otherwise, throughout the specification andclaims that follow, the word “comprise” and variations thereof, such as,“comprises” and “comprising” are to be construed in an open, inclusivesense, which is as “including, but not limited to.”

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiments,it is to be understood that the invention is not to be limited to thedisclosed embodiments and elements, but, to the contrary, is intended tocover various modifications, combinations of features, equivalentarrangements, and equivalent elements included within the spirit andscope of the appended claims.

We claim:
 1. A system for reading a data carrier that is attached to anarticle, the data carrier being responsive to electromagnetic energy(EM) of a frequency f₁ in response to which the data carrier providesunique identification data, the system comprising: a metallic enclosurehaving electrically conductive walls, the enclosure have a naturalfrequency of resonance f₂; a probe mounted through one of the walls ofthe enclosure, the probe configured to inject electromagnetic energyinto the enclosure, wherein the position of the probe in relation to thewalls of the enclosure is selected to optimize power transfer into theenclosure; a storage device located within the enclosure configured toreceive data carriers with respective articles to be stored; and areceiving antenna disposed within the enclosure and configured toreceive the unique identification data provided by the data carriers. 2.The system of claim 1 wherein: the frequency f₁ is different from thenatural frequency of resonance of the enclosure f₂; and the position ofthe probe is selected so that reflected phase of EM from the walls isequal at the probe position, i.e, they are in phase at the probeposition.
 3. The system of claim 1 further comprising an impedancematching circuit configured to more closely match impedance of the probeto impedance of the enclosure.
 4. The system of claim 3 wherein theimpedance matching circuit comprises an active impedance matchingcircuit.
 5. The system of claim 1 wherein the storage device comprises adrawer adapted to move into and out of the enclosure.
 6. The system ofclaim 5 wherein a portion of the drawer forms a portion of theelectrically conductive enclosure when the drawer is in a predeterminedposition in relation to the enclosure.
 7. The system of claim 6 whereinthe drawer is non-electrically conductive except for a portion of thedrawer that forms a portion of the electrically conductive enclosure. 8.The system of claim 6 wherein the drawer is slidable into and out of theenclosure through an opening in the enclosure, the drawer having a frontpanel that is electrically conductive and that contacts the electricallyconductive enclosure when the drawer is slid to a predetermined positionwithin the enclosure.
 9. The system of claim 1 wherein the receivingantenna is located on an opposite side of the storage device from theprobe.
 10. The system of claim 1 further comprising a plurality ofprobes, each probe being mounted through a wall of the enclosure, eachof the probes configured to inject electromagnetic (EM) energy into theenclosure at its respective position, wherein the position of each probein relation to the walls of the enclosure is selected to optimize powertransfer into the enclosure to provide coverage of a portion of theenclosure with an electromagnetic field, the position of the probesfurther selected such that the probes may be sequentially activated toinject their respective EM energy into the enclosure to provide acomposite EM field.
 11. A system for tracking an article having a datacarrier attached thereto, the data carrier being responsive toelectromagnetic energy of a frequency f₁ in response to which the datacarrier broadcasts unique identification data, the system comprising: ametallic enclosure having electrically conductive walls, the enclosurehave a natural frequency of resonance of f₂, wherein f₂ is differentfrom f₁; a probe mounted through one of the walls of the enclosure, theprobe configured to inject electromagnetic energy into the enclosure,wherein the position of the probe in relation to the walls of theenclosure is selected to optimize power transfer into the enclosure; adrawer movably disposed within the enclosure, the drawer configured tostore articles having respective data carriers; and a receiving antennadisposed within the enclosure and configured to receive the uniqueidentification data provided by data carriers.
 12. A system forestablishing a robust electromagnetic field within an enclosure, thesystem comprising: a metallic enclosure having electrically conductivewalls, the enclosure have a natural frequency of resonance f₂; a probemounted through one of the walls of the enclosure, the probe configuredto inject electromagnetic energy (EM) into the enclosure, the EM havinga frequency of f₁, wherein f₁ and f₂ are different, wherein the positionof the probe in relation to the walls of the enclosure is selected tooptimize power transfer into the enclosure; a storage device locatedwithin the enclosure configured to receive data carriers with respectivearticles to be stored; and a receiving antenna disposed within theenclosure and configured to receive the unique identification dataprovided by the data carriers.